Oh those meanies, always seeking any opportunity to make a writer's job so much harder! Okay, okay, so I'm not such a funny guy. Here's what I have always considered the differences between Means and Opportunity.
Opportunity conveys the actual, physical ability to have been able to commit the crime. The time frame is always extremely important and many individuals have been caught due solely to the "WHEN". The criminal's proximity to the victim or crime scene. As you know, most murders are committed by people known to the victim. Was there something the criminal could immediately gain by committing the crime at that particular time (see, time again), many crimes are crimes of "opportunity" such as baggage checkers at airports stealing valuables from someone's suitcase.
Means means (okay, sorry again) the criminal's ability to commit a crime as referenced by that particular criminal's skill, tools, background, etc. If a particularly breach-proof safe is broken into without causing undo damage to said safe physically, law enforcement starts looking for those unsavory characters that have the singular skills and/or access to the tools needed to have committed this act of perfidy. That criminal has the "means" to have committed his/her crime.
Anyway, that's what I think. Hope it helps.